490 



MANUAL OF ZOOLOGY. 



bodies of the vertebrae are marked out by osseous or semi- 

 cartilaginous rings, enclosing the primitive matter of the noto- 

 chord. In others, the vertebrae are like those of the Bony 

 Fishes ; that is to say, deeply bicon- 

 cave or " amphiccelous." In one Gan- 

 oid, however the Bony Pike (Lepi- 

 dostcus] the vertebral column consists 

 of a series of " opisthocoelous " ver- 

 tebras ; that is to say, vertebrae which 

 are convex in front and concave be- 

 hind. This is the highest point of 

 development reached in the spinal 

 column of any fish, and its structure is 

 more Reptilian than Piscine. In Poly- 

 pterus and Amia the vertebrae are ossi- 

 fied and amphicoelous. The remain- 

 ing existing genera have a persistent 

 notochord. 



II. The exoskekton consists in most 

 Ganoid fishes of scales, plates, or 

 spines, which are said to possess ganoid 

 characters. The peculiarities of these 

 scales are that they are composed of 

 two distinct layers an inferior layer 

 of bone and a superficial covering of a 

 kind of enamel, somewhat similar to 

 the enamel of the teeth, called " gano- 

 ine." In form the ganoid scales most 

 typically exhibit themselves as rhom- 

 boidal plates, placed edge to edge, 

 without overlapping, in oblique rows, 

 the plates of each row being often arti- 

 culated to those of the next by distinct 

 processes (fig. 247, e). In some cases, 

 Fig. 264. Skeleton of the pec- however, the scales are circular, and 

 overla P ? ne another, as in the ordinary 



Bony Fishes. Ill AdpCHSer (fig. 253, 



B) and Scaphirhynchus there are de- 

 tached dermal plates of true bone ; 

 whilst Spatularia has the skin naked. 



III. As to the fins, both pectorals and ventrals are usually 

 present, and the ventrals are always placed far back in the 

 neighbourhood of the anus, and are never situated in the im- 

 mediate vicinity of the pectorals. In some living and many 

 extinct forms the fin-rays of the paired fins are arranged so as 



gent branches on each side. 



c * rt ^ se ' (Afier 



